1. Field of the Invention
This invention relates generally to film metering systems for cameras and, more particularly, to viewfinder warning flag systems to indicate proper film metering in single-use cameras.
2. Description of the Related Art
In a photographic camera, a sequence of exposed images is produced on a roll of photographic film. After each exposed image is produced, the film is moved such that an unexposed portion of the film is located at an exposure gate. When a camera user presses a shutter release button, light from a photographic scene is permitted to pass through an objective lens onto the film under control of a shutter mechanism, thereby creating an exposed image on the film at the exposure gate. The shutter is then cocked, or readied for the next exposure. It is important for the film to be moved in the camera by precisely one frame spacing for each exposure to avoid double-exposed images, irregularly spaced exposed images, and film waste. The control of film movement in a camera to ensure correct frame-to-frame spacing between exposed images is referred to as film metering.
Conventionally, some cameras have metering mechanisms that prevent the camera shutter from being cocked and triggered if the film has not been properly moved by one frame. Such mechanisms prevent metering problems, but can add a relatively great deal of bulk and expense to the camera. This can be critical in the case of, for example, single-use cameras where size, weight, and cost considerations are paramount. Many cameras include a mechanical film exposure counter that indicates the number of exposed frames on the film roll. The mechanical structure necessary to provide the exposure counter can be relatively simple, thereby minimizing size, weight, and cost. An exposure counter helps the user to determine if the film is moving properly in the camera and acts as a check on proper metering. A user, however, can easily forget to check the exposure counter and can inadvertently continue making exposures even if the film has not advanced properly in the camera or has not been properly metered.
To prevent improper film movement from occurring unnoticed, it is known to provide film metering warning flags within the field of view of camera viewfinders. A camera user looks through the viewfinder to compose a photographic scene, and therefore a warning flag in the camera viewfinder is more likely to be noticed than other indicators of improper film movement. Many such warning flag indicators, however, have rather complicated linkages that are relatively bulky and expensive to produce. Again, this is especially troublesome with respect to single-use cameras, where size, weight, and manufacturing cost are of extreme importance.
U.S. Pat. No. Reissue 32,391 to Berg describes a viewfinder warning indicator for a photographic camera in which a series of spur gears and a release clutch are used to raise a mechanical flag into the viewfinder field of view when the film is not properly advanced. Proper film advancement causes the flag to be lowered out of the viewfinder field of view. The spur gears and release clutch can be relatively bulky and costly for application to single-use cameras. U.S. Pat. No. 3,688,668 to Ort describes a film transport indicator in a camera in which a sensing finger is linked to an indicator tab. When a camera user manually advances the film a distance of one frame, the sensing finger engages a film perforation and operates through the linkage to push the indicator tab so it extends outside of the camera housing. The presence of the indicator tab outside the camera signals the camera user to stop advancing the film. Thus, there is no warning indicator visible in the viewfinder and there is no automatic film stop.
From the discussion above, it should be apparent that there is a need for a viewfinder metering warning flag system that provides an indication of film metering without requiring complicated mechanisms and at reasonable cost, with particular application to single-use cameras. The present invention fulfills this need.